A communication device with a modulation-demodulation function provided with interfaces on both the terminal side and the network side is known as means for connecting a portable information terminal equipment such as a notebook personal computer or an electric organizer, etc., to the public telephone network. The communication device can be switched to either one of a cable channel such as a public telephone network, etc., and a radio channel such as a cellular telephone network, etc., so that either one of the channels can be selectively used. However, the described communication device has the following problems. The user must perform a troublesome operation of setting various values, for example, for altering set values in a modem, converting a protocol, etc., according to the type of a channel to be used, which results in low operability. Moreover, if the setting is not made correctly for the type of the channel to be used, in the worst case, the communication may not be performed, thereby presenting the problem of poor usability.
There is a known method of eliminating the described inconveniences, wherein a special adaptor having an adaptor identification signal output section formed therein is adopted for connecting the cable channel or the radio channel to the communication device, and by connecting thereto the adaptor, the communication device automatically identifies the type of the connected channel. As illustrated in FIG. 16, the described communication device includes a cable channel connector 55, a radio channel connector 56 and an external control terminal connector 57. To the cable channel connector 55, connected is a cable channel adaptor 52 for connecting the communication device 51 and the cable channel. To the radio channel connector 56, connected is a radio channel adaptor 53, and with the adaptor 53, the communication device 51 is connected to an analog signal input-output terminal (audio terminal) normally provided in a wireless telephone, etc. In the described manner, the communication device 51 is connected to the radio channel. To the external control terminal connector 57, connected is an external control adaptor 54 for connecting the communication device 51 and an external control terminal such as a notebook personal computer, etc.
FIG. 17 illustrates the configuration of essential parts of the communication device 51. The communication device 51 includes: a channel identifying circuit 58 for determining a type of a channel connected to the device, a Network Control Unit (NCU) 59 for controlling communications with respect to a public telephone network, a switching section 60 for switching a channel to be used, a modem 61, a CPU (Central Processing Unit) 62 for controlling the operation of the entire device, a ROM (Read Only Memory) 63 for storing therein operation programs of the CPU 62.
The CPU 62 controls the communication device 51 in the following manner. When connection channel information is requested from the external control terminal, the CPU 62 identifies the type of the channel currently connected based on a channel identification signal from the channel identifying circuit 58, and outputs a code corresponding to the connected channel to the external control terminal. Then, the external control terminal which receives the code determines various values and the protocol according to the connected channel, thereby controlling the communication device 51.
FIG. 18 is a circuit diagram schematically showing an internal circuit structure of the channel identifying circuit 58, and the channel adaptors 52 and 53. When the channel adaptors 52 and 53 are respectively connected to the connectors 55 and 56 of the communication device 51, adaptor identification signals a, b, c and d that are set in the cable channel adaptor 52 and the radio channel adaptor 53 are inputted to the channel identifying circuit 58. Further, the channel identification signals e and f according to the adaptor identification signals a, b, c and d are sent from the circuit identifying circuit 58 to the CPU 62. The adaptor identification signals a, b, c and d set in an adaptor differ between the cable channel adaptor 52 and the radio channel adaptor 53.
In the described example of the circuit, a logic wherein a high circuit level is represented by "1", and a low circuit level is represented by "0" is adopted. In the cable channel adaptor 52, the identification signal a and the identification signal b are respectively set to "1" and "0". On the other hand, in the radio channel adaptor 53, the channel identification signals c and d are respectively set to "0" and "1". In this example of the circuit, in the state where both adaptors 52 and 53 are connected, both of the channel identification signals e and f to be outputted to the CPU 62 is set to "0". In the case where neither of the adaptors 52 and 53 are connected, both of the channel identification signals e and f are respectively set to "1". On the other hand, if only the cable channel adaptor 52 is connected, the channel identification signals e and f are respectively set to "1" and "0", while in the case where only the radio channel adaptor 53 is connected, the channel identification signals e and f are respectively set to "0" and "1". The CPU 62 identifies the type of the connected channel based on the channel identification signals e and f outputted from the channel identifying circuit 58.
However, in the described arrangement, the special cable channel adaptor 52 and radio channel adaptor 53 are required for the communication device 51, and a commercially available cable or a cable which is equipped with or available as an optional component of a radio telephone machine cannot be used without modifying it. Therefore, in the aspect of the manufactures, since a cable optionally available for the communication device 51 must be developed, it takes time for the development. Moreover, in the aspect of the user, since the cable optionally available for the communication device 51 is additionally required, a high cost is incurred.
Furthermore, the channel adaptors 52 and 53 are always required for connecting the communication device 51 to the cable channel and/or the radio channel, thereby presenting the problem that the device becomes larger in size.
The Japanese Laid-Open Patent Publication No. 48771/1993 (Tokukaihei 5-48771) discloses a communication device where a public switched telephone network (PSTN) and an integrated services digital network (ISDN) are connected, and a communication is performed by selecting one of the two channels. In this communication device, the selection of the channel is performed by the input operation by the user, and both of the connected channels are cable channels. In these points, the present invention should be distinctive of the above-mentioned Publication.
The Japanese Laid-Open Patent Publication No. 125763/1987 (Tokukaisho 62-125763) discloses a branch control system by means of a DC branch device. However, the object of this system is to increase the number of control terminals to be connected to a single channel (cable channel). The publication does not refer to the automatic detection system for enabling the discrimination between the cable channel and the radio channel. Thus, the present invention should be distinctive of the described system of the publication.
The Japanese Laid-Open Patent Publication No. 129649/1989 (Tokukaihei 1-129649) discloses a detection method for the hook switch state (the state of the telephone machine connected in parallel to a data communication section for an image data communication, etc.). In this system, unlike the present invention, the state of the detecting section of the hook switch does not serve as the detecting section for the connection of the cable channel/radio channel. Thus, the present invention should be distinctive also of this system.